Essentially, the 1st show is for the folks with the first dinner seating, the 2nd show for the folks with the 2nd dinner seating. Yes, shows do overlap seatings, so if you are eating at the 2nd dinner, the 1st show will be happening during your dinner.
No, you cannot reserve seats for any of the shows.
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04